There are many situations where it is desirable to inhibit unauthorised entry to, or unauthorised use of, environments such as buildings and motor vehicles. In the case of motor vehicles, ignition keys provide a basic level of security but they are not entirely satisfactory in that they can be easily circumvented to allow the engine of the vehicle to be started. There are proposals which provide an additional level of security, such proposals incorporating another form of security device in the motor vehicle such as a valve in the fuel supply line or a switch in the electrical wiring controlling the operation of the engine. For convenience, such other security devices are generally controlled by a solenoid or other switching device such as an electronic security alarm system, which can be energised or otherwise operated from a battery in the electrical system of the vehicle. While the presence of such other security devices does afford some additional security protection to the vehicle, it is still possible for the wiring of the control circuitry of such other security devices to be rearranged so as to actuate the solenoid or other switching device thereby to disarm the security system and to allow the engine to be started. Similar considerations apply to electronically controlled security locks for doors in environments such as buildings and safes.
The security devices described above are vulnerable because they operate at voltage levels which are available from the normal power supplies derived in the environment, such as a twelve volt supply typically used in the engine operating control circuit of motor vehicles. This normal power supply can therefore be used to facilitate disarming of the security device.